RJMAZ wrote:Let's crunch some numbers.

It is highly likely that the smaller longer ranged 797-8 will be very similar to the A310-300 and 767-200ER in terms of dimensions, weights and range.

OK but let's actually crunch some numbers instead of just positing that a 798 will be of similar weight/size to A310/762.

Your analogy ignores the impact of SFC, L/D, and weight-tech (CFRP) improvements on all relevant parameters.

If we start with deltas for L/D and SFC from 762ER to a 5,000nm 798, you immediately see dramatic MTOW decrease:

- MTOW for 762ER: 295,000lbs
- Spec OEW 762ER: 181,610 lbs
- Spec range: 6,590nm

Now apply some reasonable first-cut deltas:

- -25% SFC
- +10% L/D
- Range decrease from 6,590 to 5,000nm

Do you know the Breguet range equation? Fiddling with that equation and assuming both planes 216pax@225lbs, a 5,000nm 798

*with the same OEW as 762ER* has an MTOW of ~300,000lbs.

Given -95,000lbs OEW delta, we can now reduce OEW. To estimate, let's say that the wing+engines+empennage+LandingGear = 60% of OEW.

Let's further say that each of these components is linearly proportional to MTOW (a conservative estimate given square-cube effects).

OEW delta = (95 / 395} * 0.6 * 181,610 = ~26,000lbs

Now take a second cut at Breguet Range Equation with your new OEW (same L/D and SFC) and you get MTOW = 278,000lbs.

Now we can go back and shrink OEW for our new lower MTOW. Our L/D is probably higher given smaller empennage/engine/wing size.

You get the idea, right? We can repeat this process until we settle at OEW ~150k and MTOW ~260k.

We end up with ~50% fuel efficiency delta.

RJMAZ wrote:If you went off the 757 and assumed the 797 has a maximum takeoff weight of 150T that is 30% heavier than the 757-200. The 752 has about 40,000lb of thrust. So 30% more thrust than the 757 is 52,000lb of thrust. So 50,000lb of thrust is actually spot on.

I'm not going to go through each of your analogies but just want to point out how conceptually mistaken this approach is.

Instead of comparing based on capacities and ranges, you should start with OEW+payload+mission fuel. A 1970's 5,000nm mission fuel is ~50% * [OEW+Payload]; a 2020's 5000nm mission fuel will be ~30%*[OEW+payload].

After figuring your fuel weight delta over [OEW+payload], you should look at how that fuel weight factor feeds back into your OEW figure.

Clear(er) example: A345 and A359ULR carry about the same payload on SIN-NYC. A345 has 220k lbs-T; A359 has 150k. A345 takes off at 840k lbs, A359ULR at 616,000. A359 burns 40+% less fuel.

15 years separate A345 and A359

43 or 44 years could separate 762ER from 797-8

Can we stop it with these simplistic analogies?

In case you find 50% fuel reduction ridiculous, consider that Boeing promises 30%

**economic** improvement.

When I hear Boeing say economic improvement, I assume it means economic improvement - dollars to the bottom line instead of fuel burn.

But to hit 30% economic improvement you need something in excess of 40% fuel burn delta plus smaller deltas to maintenance and (maybe) acquisition cost.

So Boeing seems to forecast the ambitious 40+% fuel delta from 767 that I see as plainly possible from the fundamentals.